This invention relates generally to radio communication systems such as CDMA systems, and more particularly to synchronization of a receiver with the received signal, which may have experienced transmission delays.
CDMA (code-division multiple access) systems are well known. See, generally, CDMA Cellular Mobile Communications and Network Security, Dr. Man Young Rhee, Prentice Hall 1998, ISBN 0-13-598418-1, and standard TIA/EIA/IS-95, hereinafter xe2x80x9cIS-95xe2x80x9d.
In CDMA systems, information bits transmitted from a mobile station to a base station may undergo convolutional encoding, symbol repetition, and block interleaving, the result of which is a bit stream comprising what are known as code symbols. As shown in FIG. 1 (prior art), the code symbols are then modulated into xe2x80x9cWalsh chipsxe2x80x9d. They are then xe2x80x9cspreadxe2x80x9d into PN (pseudo-noise) chips by sequential bits of what is known in the CDMA art as the xe2x80x9clong codexe2x80x9d (LC). The PN chips are then OQPSK (offset quadrature phase-shift keying) modulated into two channels, known as the I channel and the Q channel, the latter being delayed by one-half a PN chip duration. Spreading to the two channels is accomplished by adding the PN chip stream in modulo-2 arithmetic to sequential bits of two different pseudo-noise binary sequences known as PNI and PNQ respectively. The PN chips are then converted from baseband to a radio transmission frequency for xe2x80x9cair linkxe2x80x9d (radio transmission) to the base station.
When the base station demodulates the signal it converts back to baseband and undoes the aforementioned OQPSK modulation, long-code spreading, and Walsh modulation. To this end, counterparts of the mobile station""s Walsh functions, long code generator, PNI generator, and PNQ generator can exist in the base station in order to produce xe2x80x9clocal replicasxe2x80x9d of the codes used to spread and modulate the transmitted data. However, at the inception of transmission from the mobile station, the base station""s generators are generally not synchronized with the received signal because of, among other things, delays occurring in transmission. Also, the mobile station may introduce delay for purposes of collision avoidance. It is thus necessary for the base station to determine the amount of transmission delay in order to effect synchronization. This is done by apparatus generally known as a xe2x80x9csearcherxe2x80x9d. It can generally be assumed that the delay will be no greater than a particular amount, known as the xe2x80x9cuncertainty regionxe2x80x9d.
Conventional searchers generally operate on trial and error, trying varying amounts of delay with correlator circuits against the incoming received signal until meaningful results are obtained.
A xe2x80x9cone-branchxe2x80x9d search scheme attempts correlation against live, incoming input using only one correlator. Such a scheme is quite slow and cannot meet strict requirements for CDMA systems (as set forth in the IS-95 standard, for example).
A xe2x80x9cmultibranchxe2x80x9d scheme using live input data (essentially, a plurality of one-branch searchers, each trying a different amount of delay) are commensurately faster than one-branch schemes, but in many cases still fall short of meeting strict standards, as those for CDMA systems.
A xe2x80x9ctotal parallelxe2x80x9d search strategy optimizes searching according to the maximum-likelihood criterion. Such schemes are much faster than xe2x80x9cone-branchxe2x80x9d and meet the strict requirements for CDMA systems; however, this is obtained at the expense of much greater complexity and expense.
Passive xe2x80x9cmatched filterxe2x80x9d schemes acquire signals rapidly, since the time to cover all possible unknown positions of the uncertainty region is simply the length of the section of the PN code that is loaded as coefficients of the matched filter plus the length of the uncertainty region. However, building a matched filter of appropriate length can be quite expensive.
Accordingly there is a need for a relatively high-speed, low-cost searcher for determining transmission delay in a radio transmission system.
It is thus an object of the present invention to provide a relatively high-speed searcher for determining transmission delay.
It is another object of the present invention to provide a relatively low-complexity searcher for determining transmission delay.
It is another object of the present invention to provide a relatively inexpensive searcher for determining transmission delay.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.
In accordance with the teachings of the present invention, these and other objects may be accomplished by the present high-speed, low-cost systems and methods of determining transmission delay. An embodiment of the present invention includes A/D converters for taking samples of the received signal after conversion to baseband, buffers for storing the samples, means for generating local replicas of the mobile station""s code generators and for presenting various sequential states of the local replicas to correlators for time-compressed correlation of the local replicas with the stored samples, and determination of the transmission delay according to the relative position of a correlation value significantly greater than all the other correlation values.
The invention will next be described in connection with certain exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.